The present invention relates, in general, to the field of electric machines.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
German patent publication no. DE 42 42 132 A1 describes an electric machine having a housing for accommodation of a rotor, which is constructed in the form of a laminated rotor core and includes a rotor shaft, and a stator. Placed inside the housing is a fan, which is arranged on both ends of the rotor shaft. Cooling channels extend in the rotor core along the entire axial length thereof and convey coolant for cooling components of the electric machine inside the housing.
Electric machines of all power outputs, especially those of higher output, develop significant amounts of heat which must be removed to attain a more efficient operation and service life. Larger electric machines, such as, e.g., generators, may be equipped, for example, with a cooling device using H2 gas for cooling the stator and the rotor, as described, e.g., in the publication entitled “Proceedings of the American Power Conference”, Vol. 39, Chicago 1977, pages 255 to 269. H2 gas is hereby circulated in an encapsulated housing. This approach not only requires complicated sealing measures but also demands considerable safety measures.
Another approach involves the application of water-cooled generators which use circulation of water in channels that extend predominantly through the stator bars or stator cores. The provision of pumps is required however to effectuate the water circulation, and water needs to be conditioned to provide corrosion protection.
Air-cooled machines, such as motors, especially with an output under 300 MVA, are known to realize a cooling action by applying a comparably large air flow. This air flow is hereby conducted through a network of fine channels. Examples include the afore-mentioned German patent publication no. DE 42 42 132 A1 or European Pat. No. EP 0 823 370 A1. A shortcoming associated with this approach is generation of unwanted heat by the air stream itself as a result of friction losses.
Enclosed machines, which lack incoming coolant from outside to flow through the electrically active component of the machine, use external ventilation. This approach leads, however, to an uneven temperature profile in the motor, as has been described in the magazine “Drive and Control, number 1, 1992, pages 10 to 12. While the bearing and end winding of the stator winding on the non-driving side of the electric machine are normally subjected to temperatures below their thermal limits, the driving side, on the other hand, encounters higher temperatures which subject the bearing and end winding of the stator winding on this driving side to greater stress, ultimately causing harm to the electric machine. To address this problem, it has been proposed to balance the difference in temperature between both sides by an additional air flow to attain a better cooling action. One such approach is disclosed in German patent publication no. DE 29 51 859 C2, and involves the use of external ventilation of the machine housing. The exterior of the housing is hereby formed with cooling fins for passage of an axial air stream which is generated by a fan operated by the shaft at the end zone of the non-driving side. While heat may be transferred from the laminated stator core to the immediately adjacent surrounding outer housing, this approach is insufficient to remove enough heat from the area of the end windings which project out from the stator core at the ends thereof. Thus, there is oftentimes the need for providing a particular channel system for air cooling inside the machine housing in order to cool the rotor and the end windings. One example of this proposal is the afore-mentioned German patent publication no. DE 42 42 132 A1.
Heat produced by the rotor also leads to losses and can be removed partly by heat conduction via the surrounding air gap to the stator and ultimately from the stator to the outer machine housing, which may be connected to the stator, and to an air flow that cools the housing. The air gap represents, however, a significant heat barrier. Another part of the heat-based rotor losses is conducted to the outside via the motor shaft and thus heats up the shaft bearings. As a result, the service life of the bearing is adversely affected because the average bearing temperature and temperature differential between the typical ring-shaped bearing inner and outer parts is too high.
A major portion of these losses, in particular the area of the end windings projecting on both sides of the stator into the housing interior, can however only be transferred through convection onto the end surfaces of the machine housing. The convection can be improved by providing the end surface of the rotor with knobs or wings to cause an undefined turbulence of the air stream inside the housing. This requires however measures to prevent the end surfaces of the housing from being exposed to excessive heat because the shaft bearings at these zones cannot be subjected to temperatures beyond a set maximum value.
It would be desirable and advantageous to provide an improved electric machine to obviate prior art shortcomings and to attain a cooling action in a simple and yet reliable manner especially in the area of the end windings.